This invention relates to an improved apparatus for the detection of radon gas using an electret detector.
A known technique in the field of radon gas detection which has been found to be highly effective is the method and apparatus for detecting radon disclosed in U.S. Pat. No. 4,853,536 to Dempsey et al. for "An Ionization Chamber for Monitoring Radioactive Gas", the disclosure of which is hereby incorporated by reference. Briefly, the technique employs a small enclosed chamber with an electrostatically charged electret attached to the inner chamber wall. A filtered hole in the chamber permits radon or other radioactive gas to enter by diffusion. Positive or negative ions (depending upon the polarity of the electret charge) formed in the chamber air by decay of the radioactive gas move to and collect on the electret surface by virtue of its electrostatic attraction to ions of opposite polarity. These ions accumulate and cause a measurable reduction in the surface voltage of the electret and such a reduction is proportional to the time integrated concentration of the radioactive gas in the chamber. The electret surface voltage reduction value is used to calculate the average concentration of the radioactive gas during the exposure period. The electret thickness and chamber volume can both be increased to increase the sensitivity of the invention to radioactive gases such as radon, tritium or carbon-14 dioxide or other such radioactive gases.
Typically, this radon measuring technique is carried out by placing one or more chambers, each containing an electret detector, in locations to be monitored such as a dwelling and permitting the detectors to remain at the monitored site for a preselected period of time such as several days. After exposure, the chambers are opened and the electret surface voltage is measured to determine its reduction over the length of time monitored. The thus obtained voltage reduction is used to determine the average radon concentration at the monitored site.
While various types of electret ion chambers have been used for measuring ions in the air most tend to be bulky and difficult to use. The above-referenced radon monitor employed a chamber having a cup-like shape and further discloses a method of reducing the volume adjacent the electret when the radon monitor is not in use. With the need to measure radon concentration in dwellings and in the workplace environment, there arose a need for an easy-to-use device employing the electret detector method for radon monitoring. Such a device would be able to be turned on and off at the monitoring site from outside the device to permit accurate timed exposures without the necessity of immediately measuring the electret surface voltage before and after a test. Additionally the device would allow easy insertion and removal of the electret for surface voltage measurements.